Identity card with physical unclonable function

ABSTRACT

An identity card, comprising a card body and a physical unclonable function arranged within the card body, wherein the physical unclonable function comprises a first light influencing layer and a second light influencing layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/436,253 filed Apr. 16, 2015, which is the National Stage ofInternational Application No. PCT/EP2013/064834, filed on Jul. 12, 2013,the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The preset invention relates to identity cards with physical unclonablefunctions (PUF).

BACKGROUND

Today, a wide variety of applications—from banking to access control toelectronic passports—are based on identity cards, i.e. smart cards.These can be equipped with cryptographic modules and guarantee a highlevel of security for systems and users. However, designers andmanufacturers of smartcards and security ICs are continuously aiming atfurther enhancing their cards' security.

In order to further improve security of an identity card, PhysicalUnclonable Functions (PUFs) can be provided. PUFs are functions embodiedin physical structures which are easy to evaluate but hard to predict.Furthermore, implementations are difficult to duplicate, even if theexact manufacturing process is known and available. Consequently,security architectures based on PUFs promise inherent unclonability,physical tamper resistance and secure establishment of device-uniquecryptographic keys. Well-known bare silicon PUFs aim at ensuring thesecurity of the circuit itself but do not grant tamper resistance to thecard in which it is embedded.

The challenge-response behaviour of a PUF is determined by sub-micronphysical characteristics formed by manufacturing process variationswhich are not reproducible, not even by the original manufacturer. PUFsenable the generation of a device-unique secret based on physicalproperties rather than on binary memory mechanisms such as fuses. Duringthe last decade, several PUF constructions have been proposed andrealized as described in Lim, Daihyun and Lee, Jae W. and Gassend,Blaise and Suh, G. Edward and van Dijk, Marten and Devadas, Srinivas,Extracting secret keys from integrated circuits. In IEEE Transactions onVery Large Scale Integration (VLSI) Systems, volume 13, no 10, pages1200-1205. IEEE, 2005. Further reference is made to Thomas Esbach,Walter Fumy, Olga Kulikovska, Dominik Merli, Dieter Schuster, FredericStumpf, “A new security architecture for smartcards utilising PUFs”, tobe published in “Securing Electronic Business Processes—Highlights ofthe Information Security Solutions Europe 2012 Conference”.

PUFs are usually based on silicon structures and utilize tiny deviationsin the electronic circuitry well below the tolerances of fabricationprocesses. These non-reproducible deviations make the circuitry uniqueand are often referred to as ‘chip fingerprints’.

However, in spite of intensive research, PUF implementations remain fewand far between and are rarely considered in the context of high-endsecurity architectures.

SUMMARY

It is therefore the object of the invention to provide an efficientconcept for providing identity cards with a PUF.

This object is achieved by the features of the independent claims.Further embodiments are disclosed in the description, the figures andthe figures.

The invention is based on the finding that the above object can beachieved by arranging distinct light influencing layers at differentlevels in a transparent card body.

The light influencing layers can be formed by metal layers respectivelycomprising a pattern, e.g. a periodic pattern, of metal stripes forminga grating. A plurality of such light influencing layers arranged aboveeach other may uniquely diffract and/or refract and/or reflect incidentlight and thus form a unique fingerprint identifying the identity card.Such light influencing layers can easily be implemented whenmanufacturing an identity card, by e.g. embedding the light influencinglayers in a plastic, e.g. polymer card body, or by arranging the lightinfluencing layers between transparent laminate layers of the identitycard.

An advantage of the physical unclonable function is that it can comprisewell macroscopically defined structures with non-random distribution inthe card body which simplifies the manufacturing process and can allowoptimization of light influencing in terms of sensitivity of themeasuring system. At the same time the uniqueness of each fabricatedstructure and correspondingly of its optical response can be provided bythe smallest tolerances of the fabrication process on the microscopiclevel and thus is intrinsic to those PUFs.

According to a first aspect, the invention relates to an identity card,comprising a card body and a physical unclonable function arrangedwithin the card body, wherein the physical unclonable function comprisesa first light influencing layer and a second light influencing layer.

The physical unclonable function is arranged to provide an opticalfingerprint of the identity card or a kryptographic key. Therefore, thecorresponding material structure can have at least partially the opticalproperties that are different from the card body, for example a lighttransmittance which is smaller than the light transmittance of the cardbody, with regard to white light or ultra violet light or infraredlight. The card body can be light transparent and may be composed of oneor more plastic layers such as polymer layers, in particular laminatedplastic layers.

The first light influencing layer and the second light influencing layercan respectively have planar surfaces.

The light influencing introduced by the respective light influencinglayer can be a reduction of light transmittance with respect to the cardbody or an amendment, e.g. increase or decrease of reflectivity,refraction or diffraction with respect to the card body. The lightinfluencing can comprise scattering, diffracting, refracting, reflectingand/or transmittance reduction.

According to an embodiment, the first light influencing layer and thesecond light influencing layer have light transmittances which aresmaller than a light transmittance of the card body. The first lightinfluencing layer and the second light influencing layer can be e.g.refractive layers and/or diffractive layers and/or reflective layersand/or at least partially opaque layers. Thereby, an optical fingerprintof the identity card can be obtained.

According to an embodiment, the respective light influencing/influencinglayer is an optically linear layer or an optically nonlinear oroptically excited layer.

According to an embodiment, the optically linear layer is one of thefollowing layers: a metal layer or a printing ink layer or ananoparticle layer or a layer having a light refraction index which isdifferent, e.g. greater or smaller, than a light refraction index of thecard body, or a diffractive layer, wherein the optically nonlinear layeris a nonlinear material layer, and wherein the optically excited layeris a fluorescent material layer.

According to an embodiment, the first light influencing layer and thesecond light influencing layer are arranged above each other, e.g.exactly each other within a tolerance, e.g. 1% or 5% or displaced withrespect to each other.

According to an embodiment, the first light influencing layer and/or thesecond light influencing layer comprises spaced surface elements, inparticular stripes or rectangles or circular elements or oval elements.

The term “surface elements” referred to herein denotes according to someimplementations flat elements having a thickness smaller than distancebetween the layers. The surfaces can be planar surfaces. Unlikeparticles, the surfaces of the surface elements are formed by surfaceareas which can be directed towards e.g. a light source arranged on thecard body.

According to an embodiment, the first light influencing layer comprisesa pattern, in particular a periodic pattern or a non-periodic pattern ora grating, of spaced surface elements having smaller transmittance thanthe card body, and wherein the second light influencing layer comprisesa pattern, in particular a periodic pattern or a non-periodic pattern ora grating, of spaced surface elements having smaller transmittance thanthe card body.

According to an embodiment, the respective pattern is one-dimensional ortwo-dimensional. The respective one-dimensional pattern may comprise aplurality of stripes arranged next to each other so as to form a patternstructure. The respective two-dimensional pattern may comprise aplurality of areas e.g. periodically arranged so as to form atwo-dimensional grid. The respective pattern can be periodic so as toform a grating, or non-periodic, forming e.g. a non-periodic 2Dstructure which can be provided by dots, in particular ink dots.

According to an embodiment, the first light influencing layer comprisesa first periodic pattern of spaced surface elements, wherein the secondlight influencing layer comprises a second periodic pattern of spacedsurface elements, and wherein the first periodic pattern of spacedsurface elements and the second periodic pattern of spaced surfaceelements are arranged above each other. The spacing between the spacedsurface elements can have a wavelength-scale dimension.

According to an embodiment, the spaced surface elements of the firstlight influencing layer and the spaced surface elements of the secondlight influencing layer are arranged exactly above each other within atolerance, e.g. 1%, 2% or 5% of a surface width, or wherein the firstperiodic pattern and the second periodic pattern are displaced withrespect to each other.

The respective the spaced surface elements can be of equal size or canhave different size. Moreover, a distance between the respective spacedsurface elements or a lateral displacement can be equal on can vary.According, in the case of periodic structures, the respective periodscan be equal or can be different. Moreover, the spaced surface elementscan be formed by dots, stripes, rectangular areas, circular areas oroval areas.

The spaced surface elements of the first light influencing layer havecan have smaller or bigger or wider or thinner surfaces than the spacedsurface elements of the second light influencing layer.

According to an embodiment, the surfaces of the spaced surface elementsof the first light influencing layer have equal or different dimensionsthan surfaces the spaced surface elements of the second lightinfluencing layer.

Moreover, the surface elements of the first light influencing layer mayextend over the surface elements of the second light influencing layer.According to some implementations, the surface elements of the firstlight influencing layer are not completely covered by surface elementsof the second light influencing layer so that light emitted towards thesecond light influencing layer from a surface of the card body can atleast partially reach the first light influencing layer when beingarranged behind the second light influencing layer.

According to an embodiment, the respective light influencing layercomprises spaced surface elements respectively having surface orthicknesses dimensions comparable to the wavelength, or wherein therespective light influencing layer comprises spaced surface elementsrespectively spaced apart by a wavelength-scale distance, or wherein adistance between the first light influencing layer and the second lightinfluencing layer is of wavelength scale.

The term “wavelength scale” referred to herein refers to a magnitude ofone or several, e.g. 10, wavelengths.

According to an embodiment, the first light influencing layer and thesecond light influencing layer jointly form an optical lens, inparticular an optical microlens.

According to an embodiment, the card body comprises laminatedtransparent layers, wherein the first light influencing layer isarranged between successive two laminated transparent layers of the cardbody, and wherein the second light influencing layer is arranged betweensuccessive two laminated transparent layers of the card body.

According to an embodiment, the identity card further comprises a lightsource for emitting light towards the first light influencing layer andthe second light influencing layer, and an optical sensor for sensinglight from the first light influencing layer and the second lightinfluencing layer in response to the emitted light. The light source andthe optical sensor are arranged on a surface of the card body such thatthe second influencing layer is closer to the optical sensor than thefirst influencing layer.

According to an embodiment, the identity card comprises an integratedcircuit comprising the optical sensor. The integrated circuit may beformed by an ID chip arranged on a surface of or in the card body.

According to an embodiment, the identity card is an identity document ora smart card or a credit card.

According to some implementation forms, a card body area around theoptical sensor or the integrated circuit can be less transparent thanother card body areas. Thereby, the optical sensor or the integratedcircuit can be more effectively protected.

According to an embodiment, the identity card is at least partiallyformed by a transparent polymer. The identity card can have a pluralityof laminated polymer layers.

According to an embodiment, the first light influencing layer is formedby a first arrangement of light reflecting elements being arranged in afirst card body area, and the second light influencing layer is formedby a second arrangement of light reflecting elements being arranged in afirst card body area.

The reflectivity of the respective arrangement of light reflectingelements is determined as a fraction of reflected light by therespective arrangement of light reflecting elements. Preferably, allarrangement of light reflecting elements have reflectivities exceeding apredetermined threshold to ensure that even the most distant arrangementof light reflecting elements can reflect enough light to be sensed on asurface of the card body by the sensor. Thus, the requiredreflectivities of the arrangements of light reflecting elements can bedetermined in dependence on a sensitivity or resolution of an opticalsensor sensing the reflected light on a surface of the card body.

The physical unclonable function is arranged to provide an opticalfingerprint of the identity card or a kryptographic key. The card bodycan be light transparent and may be composed of one or more plasticlayers such as polymer layers, in particular laminated plastic layers.

The arrangement of light reflecting elements can be formed by metallayers respectively comprising a periodic pattern of metal stripesforming a grating. A plurality of such arrangement of light reflectingelements arranged above each other may uniquely diffract and/or refractand/or reflect incident light and thus form a unique fingerprintidentifying the identity card. Such arrangement of light reflectingelements can easily be implemented when manufacturing an identity card,by e.g. embedding the arrangement of light reflecting elements in aplastic, e.g. polymer card body, or by arranging the arrangement oflight reflecting elements between transparent laminate layers of theidentity card.

According to an embodiment, the second arrangement of light reflectingelements is arranged closer to at least one common surface point on asurface of the card body than the first arrangement of light reflectingelements.

According to an embodiment, the identity card comprises onlyarrangements of light reflecting with reflectivities being greater thanor equal to a predetermined reflectivity threshold, e.g. 5% or 10% themaximum reflectivity. Accordingly, the second arrangement of lightreflecting may have a reflectivity of 100%.

According to an embodiment, the identity card comprises only Narrangements of light reflecting elements, each of the N arrangements oflight reflecting elements being arranged in a different card body areaat a different distance from a common surface point on a surface of thecard body, and wherein a light reflectivity of the arrangements of lightreflecting elements increases, in particular linearly or along agradient, with increasing distance of the arrangements of lightreflecting elements from the common surface point.

According to an embodiment, the light reflecting elements of the firstarrangement of light reflecting elements and of the second arrangementof light reflecting elements are respectively formed by reflectiveparticles being arranged to form the respective arrangement of lightreflecting elements, or by planar surface elements, in particular metalstripes or metal rectangles or metal circular elements or metal ovalelements, being arranged to form the respective arrangement of lightreflecting elements.

According to an embodiment, a density of the light reflecting elementsof the first arrangement of light reflecting elements in the first cardbody area is greater than a density of the light reflecting elements ofthe second arrangement of light reflecting elements in the second cardbody area.

According to an embodiment, the light reflecting elements of the firstarrangement of light reflecting elements have bigger reflective surfacesthan the light reflecting elements of the second arrangement of lightreflecting elements.

According to an embodiment, the intensities of light reflected by thefirst arrangement of light reflecting elements and the secondarrangement of light reflecting elements at a common surface point on asurface of the card body are identical within an identity range.

According to an embodiment, the light reflecting elements of therespective arrangement of light reflecting elements are statistically orregularly arranged within the respective card body area.

According to an embodiment, the light reflecting elements of the firstarrangement of light reflecting elements and of the second arrangementof light reflecting elements respectively form reflective layers.

According to an embodiment, the reflective layers are planar reflectivelayers arranged at different distances with respect to a surface of thecard body, or wherein the reflective layers are arranged alongcurvatures, in particular spherical curvatures, with different radiusestowards a common center point, the common center point being a commonsurface point on a surface of the card body or being above or beneaththe surface of the card body.

According to an embodiment, the wherein the light reflecting elements ofthe first arrangement of light reflecting elements and the lightreflecting elements of the second arrangement of light reflectingelements are respectively arranged to form a periodic pattern, inparticular a grating.

According to an embodiment, a difference between the reflectivity of thefirst arrangement of light reflecting elements and the secondarrangement of light reflecting elements is greater than or equal to apredetermined threshold. The predetermined threshold can be e.g.determined by a fraction resulting from dividing 10%% by a number ofarrangement of light reflecting elements. However, the predeterminedthreshold can be set to 5% or 10% the reflectivity of the mostreflective arrangement of light reflecting elements. Moreover, thethreshold can be defined by the light sensitivity or light resolution ofan optical sensor which may sense the reflected light.

According to a second aspect, the invention relates to method formanufacturing an identity card, the method comprising providing aplurality of transparent laminate layers, arranging a first lighttransmittance between successive transparent laminate layers andarranging a second light transmittance between successive transparentlaminate layers to obtain a physical unclonable function, and laminatingthe transparent laminate layers and the physical unclonable function toobtain the identity card.

Further method steps of the method for generating an identity card aredirectly derivable from the features of the identity card.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments will be described with respect to the accompanyingfigures, in which:

FIG. 1 shows an identity card with PUF;

FIG. 2 shows an identity card;

FIG. 3 shows a stack of light influencing layers;

FIG. 4 shows intensities of light;

FIG. 5 shows a schematic diagram of a stack of three light influencinglayers; and

FIGS. 6a, 6b, 6c show the light influencing layers.

DETAILED DESCRIPTION

The identity card 100 comprises a card body 101 with a PUF 101 which isarranged within a card body and forms a probed physical structure. Thecard body is not explicitly shown in FIG. 1.

The identity card 100 further comprises an integrated circuit 103, e.g.a card chip, for integrating the circuitry for the PUF measurements,pre- and post-processing and the memories and applications into acomplex material structure which is stimulated and measured by theenclosed optional integrated security function 105. The integratedsecurity function 105 is configured to implement an application such asencryption or protocols.

The integrated circuit 103 further comprises an optional memory 107coupled to the integrated security function 105, and an interface 109for communicating with the integrated security function 105.

The integrated circuit 103 further implements an control function 111for controlling the PUF 101, wherein the control function 111 e.g.comprises an optional pre-processing 113, an optional challengeprocessing 115, and a stimulus 117 for stimulating the PUF 101 e.g. withlight. The stimulus 117 may comprise a light emitting element such as aLED or a LED array.

The control function 111 further comprises one or more sensors 119 forsensing light from the PUF 101 in response to the stimulus generated bythe stimulus 117, an optional response processing 121 and an optionalpost-processing 121.

As shown in FIG. 1, the integrated circuit 103, e.g. a security chip, isconnected with the card body both physically and logically so that theintegrated circuit 103 is able to verify the integrity of itssurroundings.

According to some embodiments, the integrated security function 105 mayoutput a Call PUF signal towards the pre-processing 113 which inresponse thereto may generate output start and challenge signals towardsthe challenge processing 115 to trigger the stimulus 117 to e.g. emitlight towards the PUF 101.

The light response from the PUF 101 is sensed by the sensor 119providing a sensor signal to the response processing 121. The responseprocessing 121 generates in response thereto the response and readysignals which are provided to the post-processing 123. The postprocessing 123 which provides a get secret signal to the integratedsecurity function 105 for e.g. encryption.

According to some embodiments, a device-unique material structureforming the PUF as e.g. key storage or optical fingerprint can be usedwhich can have multiple advantages. Because the material carries thesecrets, it is possible to bind a system to a physical object, which isone of our primary goals. Furthermore, invasive influencings wouldprobably change or destroy the material and thereby render the embeddedsecret useless.

For the scenario of an identity card as a smartcard, the choice of anoptical PUF is favourable. Optical measurements are robust againstElectro-Magnetic Interference (EMI) and in addition not only enable theuse of the card body as the physical structure to be measured, but alsoprovide the possibility of involving the printing on the card. However,resource constraints in smartcards, e.g. limited power consumption aswell as static placement of the optical measurement mechanisms, do notallow the reading of optical PUFs by modulating, moving or tilting thelaser beams.

One way to generate several different light stimuli by or within theintegrated circuit 103 is the integration of an LED array in thestimulus 117.

In general, the challenge-response behaviour of the PUF 101 can becomplex in order to resist model-building attacks and to provide enoughentropy for the desired applications.

According to some embodiments, the controlled PUF 101 is integrated inorder to protect against modelling attacks and ensure secure usage ofthe PUF 101, e.g. for key embedding and reconstruction. Following theprinciples of a controlled PUF, not only does the material enclosureprotect the inner circuits, but the control circuits also protect thesystem from read-out or other methods of unauthorized access to the coreof the PUF 101 which could enable modelling attacks. Because of this, acomponent having access to the challenge-response interface of the PUF101 can be the control logic circuitry implementing the control function111. All other application logics on the identity card can communicatewith the control logics in order to make use of the functionality of thePUF 101. As a result, mutual protection of control logic circuits andPUF structure can be achieved.

FIG. 2 shows an identity card 200, comprising a card body 201 and aphysical unclonable function 203, 207 arranged within the card body 201,wherein the physical unclonable function comprises a first lightinfluencing layer 203 and a second light influencing layer 207.

The first light influencing layer 203 comprises spaced surface elements205, e.g. metal stripes, being arranged to form a one dimensionalperiodic pattern (grating). Accordingly, the second light influencinglayer 207 comprises spaced surface elements 209, e.g. metal stripes,being arranged to form a one dimensional periodic pattern (grating). Thefirst light influencing layer 203 and the second light influencing layer207 are arranged above each other within the card body 201, which ismore transparent than the spaced surface elements 205, 209. The cardbody 201 can be formed from a transparent polymer.

The surface elements 205, 209 have dedicated surfaces being directedtowards the optional light source 211, i.e. stimulus, arranged e.g. inthe card body 201. The light source 211 may comprise an LED or a LEDarray. Thus the surface elements 205, 209 form light barriersinteracting with the light, e.g. reflecting, refracting or diffractinglight emitted by the light source 211 towards the surface elements 205,209.

The identity card further comprises a sensor 213 for sensing light fromthe surface elements 205, 209 in response to the light emitted by thelight source 211.

The light source 211 and the sensor can be integrated within anintegrated circuit 215, which may have the architecture as shown in FIG.1.

The first light influencing layer 203 and the second light influencinglayer 207 collectively form the PUF as a probed physical structurearranged in a probed volume V_(p).

The maximum amount of information that can be extracted from a PUFincreases with the space that can be measured e.g. by the chip, i.e. theintegrated circuit 215. The card area that can be accessed by themeasurement is the probed volume V_(p).

According to some embodiments, the size of the probed area is defined onthe one hand by chip specifications—such as measurement sensitivity andrelative positions of light sources and sensors—and on the other hand bythe influencing properties of the card materials. The optical propertiescan be optimized in terms of both the sensitivity of the sensors and thedesired penetration depth. A parameter which can be considered is theback-influencing of light, since the sensors on the chip measure onlythe light that is reflected or scattered back to the integrated circuit215. This can be demonstrated using the one-dimensional model structureconsisting of N equal partially reflective layers, each with reflectionR. Intensities of light emitted and reflected by a single layer n aregiven by formula I_(tn)=(1−R)I_(t(n-1)) and I_(rn)=RI_(t(n-1))respectively, where I_(t(n-1)) is intensity of light emitted by theprevious layer. The total intensity of light reflected by the stack of klayers is given by formula

$I_{rk} = {\sum\limits_{n = 1}^{k}{I_{rn}.}}$

FIG. 3 shows a stack of k (k=1/N) light influencing layers 301collectively forming a PUF. The light influencing layers 301 may bereflective layers respectively comprising a grating of surface elementssuch as metal stripes.

FIG. 4 depicts the intensities of light emitted and reflected by thestack of k (k from 1 to N) light influencing layers 301 as well as theintensity of light reflected by a single layer n. In particular, FIG. 4shows normalized intensities of light transmitted 401 (solid line),reflected 403 by the stack of k layers (circles) and reflected 405 by asingle layer n (triangles).

The contribution from a single layer decreases with the layer number n.For example, if a single layer reflects 10% of light (R=10%), themaximum contribution from a single layer to the signal does not exceed10% and the contribution from the 24^(th) layer becomes smaller than 1%.The contribution from the remaining N-24 layers does not exceed 8% ofthe total signal. This model clearly shows the relation betweensensitivity of sensor, material structure and the maximum size of theprobed physical structure.

The maximum amount of information that can be obtained from a probedarea with an optical PUF can be roughly estimated, assuming that thesmallest material structure that can be resolved by optical measurementhas a size comparable to the wavelength λ of the probing light. Theprobed volume V_(p) can be divided into N_(e) elementary volumes λ³. Inthe simplest case, each elementary volume represents 1 bit ofinformation and the maximum amount of information is given by the numberof elementary volumes N_(e)=V_(p)/λ³.

For example, from a volume V_(p) of size 1.0×1.0×0.3 mm³ probed by awavelength of 700 nm, one can extract a maximum of 8.7×10⁵ bits.

With regard to light propagation and in order to describe the propertiesof light distribution at the exit surface of the medium, the influenceof fabrication tolerances on the resulting light distribution and inorder to relate the optimum medium structure and size to the integratedmeasuring means, the theoretical light propagation in the designed mediacan be calculated.

The calculation approach can, by way of example, be based on elementarystructures. A disordered structure is mathematically resolved into afinite series of elementary periodical structures. Light propagation insuch structures can be numerically calculated in a volume compared tothe probed volume V_(p). Any single parameter of the structure, like thenumber of layers, period, contrast or spatial shift, can beindependently varied and the resulting change in light distribution canbe calculated.

FIG. 5 shows a schematic diagram of a stack of three light influencinglayers 501, 503, 505, respectively comprising spaced surface elements507, 509, 511, e.g. metal stripes, being arranged to form a periodicpattern (grating). The grating can be characterized by modulatedrefraction and absorption n(x), Δ(x), period P_(x), layer distance d anda relative layer shift Δa.

In order to determine the arrangement and/or structure of the lightinfluencing layers 501, 503, 505, the so called stitch method can beemployed. According to the stitch method, a volume is divided in aplurality of sub-volumes with e.g. 30×30×30 μm3 or elementarystructures. The light distribution can then be determined for eachsub-volume or elementary structure

The light influencing layers 501, 503, 505, are arranged above eachother, with or without a displacement in x direction. Furthermore, thesurfaces of the spaced surface elements 507, 509, 511 can be aligned toshow in the same direction, i.e. having normal showing in thez-direction and/or can be inclined with normal showing in thez-x-direction.

FIG. 6a shows the light influencing layers 501, 503, 505 shown in FIG.5, which are arranged within a transparent card body 601 of an identitycard 600. The identity card further comprises a plurality of lightsources 603 and a plurality of light sensors 605. FIGS. 6b and 6c showcorresponding light distributions obtained for reflective layerthicknesses of 10 nm, FIG. 6b , and 20 nm, FIG. 6 c.

The light influencing layers 501, 503, 505 can comprise spaced surfaceelements being formed by thin (compared to the distance between thelayers) metal stripes embedded in a transparent polymer material formingthe card body 601. The metal stripes can have different, e.g. staggered,widths as shown in FIG. 6a . The cross section of the structure is shownschematically in FIG. 6a . The period P_(x) between subsequent spacedsurface elements can set to 50 μm (wavelength-scale), and the distancebetween the light influencing layers 501, 503, 505 to about 100 nm(wavelength-scale).

The thickness of the light influencing layers 501, 503, 505 can bevaried between 10 and 30 nm or even 300 nm. The volume can beilluminated with parallel light of a wavelength of 630 nm under normalincidence.

The graphs of FIGS. 6b and 6c show the resulting spatial distributionsof light intensity normalized to the intensity of incident light. Thereis a difference in the light distribution for the different metal layerthicknesses of 10 and 30 nm. The latter results in a focussed light thusin even a further improved contrast of the light pattern. Furthermore,the light is redistributed by the structure in the entire calculatedvolume while the distribution changes with distance from the structure.The distribution achieves higher contrast at some distance, while in thearea close to the structure the contrast and the characteristic size ofthe light spots remain smaller. The results show how the lightdistribution is affected by a 20 nm difference in layer thickness.

In the following, the material selection for the light influencinglayers will be addressed.

The light influencing layers can have a different light characteristic,e.g. transmittance, then the card body surrounding the light influencinglayers. According to some embodiments, the light influencing layers canbe structured using known card production techniques, includingprinting, spraying, dispersing, embossing or vacuum depositiontechniques as well as holographic recording.

Most optical materials do not interact with light, thus providingsimultaneous and linear response. The signal from a PUF based on suchmaterials is measured by a sensor at the same moment as the light isemitted by a light source and its value is proportional to the intensityof the probe light.

Under the assumption that sensors integrate light over time t_(m), thetype of suitable materials can be extended by material that can interactwith light within time<t_(m). Optically non-linear materials changetheir optical properties under irradiation. This change is stronglydependent on the light intensity, is induced within a very short timeand relaxes after the light is switched off. Fluorescent materialsabsorb light in one spectral region and emit it in another, usuallyred-shifted, spectral region. Emission follows absorption within a veryshort time and depends strongly on the molecular surroundings of theemitting units. Typical emission times are within the range of 0.5 to 20ns.

Of particular interest are time-resolved or time-delayed measurements,where the response is measured with some delay relative to the probelight flash. An approach of this kind only makes sense if the materialresponse changes reversibly during or after irradiation. Photochromicmaterials are examples of such materials. Under irradiation with actiniclight, the photochromic unit undergoes a transformation to its otherform, which has a different absorption spectrum. Relaxation to theinitial form occurs thermally or under irradiation. The most effectivephotochromic materials are stilbenes, spiropyranes, azobenzenes as wellas bacteriorhodopsin.

The material types and their related properties are summarized in Table1.

TABLE 1 Material overview and properties Material types Relatedproperties Optically linear: Simultaneous response: printing inks,metallic response depends on light intensity; broadband inks, dispersedmicro absorption spectra; probing with different and nanoparticles,wavelengths is desirable in order to increase materials with highvariation of PUF response. refractive indices Optically nonlinear:Simultaneous response: variety of nonlinear response changes with time,but the response polymers, time is very short (shorter than measurementbacteriorhodopsin time); response depends on light intensity; nonlineareffects require very high light intensities or electrical fields.Optically excited: Simultaneous response (μs): fluorescent materialsresponse is spectrally shifted relative to the excitation; typicalexcitation in UV or blue region of VIS; requires sensor sensitivity indifferent spectral regions. Optically or thermally Time-resolvedresponse: excited: photochromic change their colour under irradiation orheat; polymers, thermochromic response depends on light intensity (orheat); polymers response time varies widely (from ms to minutes)depending on material structure; relaxation may last even longer;typically require irradiation in UV or blue region of VIS; responsedepends on the modulation frequency. Optically excited: Time resolvedresponse (seconds to hours): phosphorescent materials very longrelaxation time.

Regarding the light generation and measurement, the integration of lightsources and sensors into the security chip (integrated circuit) that isembedded in the card can be performed.

According to some embodiments, the structure of the PUF can be used forgenerating an encryption key, wherein authentication by achallenge-response protocol directly using the unique mapping ofchallenges to responses of a PUF can be provided.

According to some embodiments, material-based PUFs can be used where themeasurement circuitry is integrated into the smartcard controller andthe PUF structure is part of the card material surrounding the chip.Such PUF system is considered resistant against laser fault injectionattacks and micro probing, since any physical change results indifferent measurement data and therefore in altered PUF responses.

The invention claimed is:
 1. An identity card, comprising: a card body;a physical unclonable function arranged within the card body, whereinthe physical unclonable function comprises a first light influencinglayer and a second light influencing layer, said first light influencinglayer and said second light influencing layer being arranged within thecard body; a light source for emitting light towards the first lightinfluencing layer and the second light influencing layer; and an opticalsensor for sensing light from the first light influencing layer and thesecond light influencing layer in response to the emitted light.
 2. Theidentity card of claim 1, wherein the first light influencing layer andthe second light influencing layer have light transmittances which aresmaller than a light transmittance of the card body.
 3. The identitycard of claim 1, wherein the respective light transmittance layer is anoptically linear layer or an optically nonlinear or optically excitedlayer.
 4. The identity card of claim 3, wherein the optically linearlayer is one of the following layers: a metal layer or a printing inklayer or a nanoparticle layer or a layer having a light refraction indexwhich is different than a light refraction index of the card body, or adiffractive layer, wherein the optically nonlinear layer is a nonlinearpolymer layer, and wherein the optically excited layer is a fluorescentmaterial layer.
 5. The identity card of claim 1, wherein the first lightinfluencing layer or the second light influencing layer comprises spacedsurface elements, in particular stripes or rectangles or circularelements or oval elements.
 6. The identity card of claim 1, wherein thefirst light influencing layer comprises a pattern, in particular aperiodic pattern or a non-periodic pattern or a grating, of spacedsurface elements having smaller transmittance than the card body, andwherein the second light influencing layer comprises a pattern, inparticular a periodic pattern or a non-periodic pattern or a grating, ofspaced surface elements having smaller transmittance than the card body.7. The identity card of claim 6, wherein the respective pattern isone-dimensional or two-dimensional.
 8. The identity card of claim 1,wherein the first light influencing layer comprises a first pattern ofspaced surface elements, wherein the second light influencing layercomprises a second pattern of spaced surface elements, and wherein thefirst pattern of spaced surface elements and the second pattern ofspaced surface elements are arranged above each other.
 9. The identitycard of claim 8, wherein the spaced surface elements of the first lightinfluencing layer and the spaced surface elements of the second lightinfluencing layer are arranged exactly above each other within atolerance; or wherein the first pattern and the second pattern aredisplaced with respect to each other.
 10. The identity card of claim 8,the surfaces of the spaced surface elements of the first lightinfluencing layer have equal or different dimensions than surfaces ofthe spaced surface elements of the second light influencing layer. 11.The identity card of claim 1, wherein the respective light influencinglayer comprises spaced surface elements respectively having awavelength-scale surface dimensions or thicknesses, or wherein therespective light influencing layer comprises spaced surface elementsrespectively spaced apart by a wavelength-scale distance, or wherein adistance between the first light influencing layer and the second lightinfluencing layer is of wavelength scale.
 12. The identity card of claim1, wherein the first light influencing layer and the second lightinfluencing layer jointly form an optical lens.
 13. The identity card ofclaim 1, wherein the card body comprises laminated transparent layers,wherein the first light influencing layer is arranged between successivetwo laminated transparent layers of the card body, and wherein thesecond light influencing layer is arranged between successive twolaminated transparent layers of the card body.
 14. A method formanufacturing an identity card, the method comprising: providing aplurality of transparent laminate layers; arranging a physicalunclonable function within the plurality of transparent laminate layers,wherein the physical unclonable function comprises: a first lightinfluencing layer, a second light influencing layer, a light source foremitting light towards the first light influencing layer and the secondlight influencing layer, and an optical sensor for sensing light fromthe first light influencing layer and the second light influencing layerin response to the emitted light; and laminating the plurality oftransparent laminate layers and the physical unclonable function toobtain the identity card.